Superconducting magnets for magnetic density separation: A NbTi based demonstrator

The presented work concerns the design of a superconducting magnet for use in Magnetic Density Separation (MDS). This magnet is being constructed at the University of Twente and will serve in a demonstrator set-up for the separation of shredded electronic materials. MDS is a novel separation technology, that can be used in for example the recycling industry. The technique is based on the combination of a fluid that is strongly attracted by magnetic fields (a ferrofluid) and a magnetic field with a strong gradient in a single direction. When shredded non-magnetic particles are inserted in the fluid bed, they will move towards different stable depths in the fluid that corresponds to their mass density. This means the MDS process can separate multiple densities in one single step, for example different plastics or electronics. State-of-the-art MDS systems use permanent magnets. Compared to these magnets, superconductors can generate a stronger magnetic field, increasing the upwards force. This allows the separation of dense particles. Another advantage that comes with the stronger magnetic field is that a lower concentration of magnetic nanoparticles in the fluid can be used while maintaining a strong upwards force. This reduces operation expenditure, because the ferrofluid is expensive and must be regularly replenished due to post-processing losses. A second advantage in using superconducting magnets for MDS results from the fact that the separation resolution scales linearly with the pole size of the magnet. Electromagnets can use a wider pole size than permanent magnets and thus an enhanced separation resolution is possible. The work involves the design and construction of the first superconducting magnet for use in magnetic density separation. The magnet consists of a conduction-cooled set of three NbTi-based racetracks, providing a gradient of 20 T/m at the bottom of the fluid bed. Electromagnetic-, mechanical- and thermal design aspects are covered. The main MDS specific design challenge was to minimize the distance between the coils and the fluid. The thesis also estimates the potential performance of future high-field MDS magnets